As the earth's energy supplies, particularly of petroleum and natural gas are becoming scarcer, increasing importance is being attached to the use of organic waste materials and regenerative raw materials, such as wood, straw and other agricultural products.
Therefore, work has been carried out again on processes for the saccharification of wood by hydrolysis of cellulose to glucose because from the latter ethanol can be reduced, which can be added to gasoline and diesel fuel. Such wood saccharification processes, particularly of the type described in German Pat. No. 577,850 were to a limited extent used on an industrial basis up to the end of World War II and more recently.
It is now known on the basis of theoretical research from the article by Hans E. Grethlein in the Journal "Biotechnology and Bioengineering", Vol. XX, 1978, pp. 503 to 525 "Comparison of the Economies of Acid and Enzymatic Hydrolysis of Newsprint" that a high glucose yield, based on the .alpha.-cellulose used is obtained if hydrolysis temperatures of 250.degree. to 300.degree. C. are raised at pressures of 40 to 90 bar and if dilute sulphuric acid with a concentration of up to 2.0% is used, accompanied by an extremely short hydrolysis time of up to 0.03 sec. However, such hydrolysis conditions have not hitherto been industrially attainable.
It is known from DAS No. 1,642,534 to comminute cellulose-containing raw materials to dust particle size and to mix them with water and dilute sulphuric acid until a pumpable mixture is obtained. By means of a suspension pump, this pumpable mixture is pumped through a pipe system with different sections. The suspension is firstly indirectly heated to hydrolysis temperature in a heat exchanger over heating surfaces. Hydrolysis then takes place in a thermally insulated reaction vessel for a period of up to 6 minutes. The hydrolyzed product is then cooled in a heat exchanger. In addition, the pressure in the system must be regulated. This known apparatus and process is not suitable for obtaining very short residence times, so that the glucose yields which can be obtained are not high. In addition, the specific energy use is very high, because all the water of the suspension must be heated to hydrolysis temperature. The suspension pump must carry out all the mechanical delivery work in the high pressure area and this is completely destroyed in the regulating valve during release. Waste heat recovery is only possible to a very limited extent.
DOS No. 2,994,789 describes a fundamentally similar process with a similar apparatus in which heating takes place by steam injection. The disadvantages described hereinbefore are substantially the same.
German Pat. Nos. 1,567,350 and 1,567,335 describe percolator fixed bed reactors for a semicontinuous hydrolysis in which dilute sulphuric acid is gradually trickled over a fixed bed of wood chips and in which cellulose is reacted to glucose with a yield of approximately 50% at a hydrolysis temperature of 120.degree. to 145.degree. C. and a residence time of 15 to 60 minutes. Quite apart from the relatively low glucose yield, a high specific energy use is required.